Process for treatment of wood using a carrier fluid under high pressure without damaging the wood

ABSTRACT

A process for treatment of wood susceptible to damage and having a length that is over the critical length with a fluid under high pressure is disclosed. The process is conducted in a way so that the fluid is not allowed to enter into the liquid form inside the wood. In one embodiment the process is performed in a way so the temperature during the pressurizing is not allowed to exceed the plastification temperature of the wood. In this embodiment the rate of damages of the wood is even further reduced.

The invention relates to treatments of wood with a carrier fluid underhigh pressure conditions, in particular supercritical conditions. Morein particular the invention relates to measures and procedures to betaken in order to avoid damage of the wood during the treatment with asupercritical fluid, preferably carbon dioxide.

BACKGROUND FOR THE INVENTION

Carrier fluids under high pressure, such as under supercriticalconditions, are increasingly employed in wood treatment processes forimpregnative or extractive purposes. Fluids under high pressure haveproperties partly similar to both gases and liquids. Thus thepenetration properties of supercritical fluids are similar to gaseswhereas the solubilisation properties are similar to liquids.

Carbon dioxide is a very attractive compound for use as a supercriticalmedium for treatment of wood because of a suitable critical point (31°C., 73 bar), a low chemical reactivity and a low toxicity. Furthercarbon dioxide is available in large quantities at a relative low cost.

An article of Morrell and Levien: “Development of New TreatmentProcesses for Wood Protection” Conference report from “Conference onWood Preservation in the '90s and Beyond” Savannah, Ga., USA, Sep.26-28, 1994, deals with impregnation of wood species normally resistantto impregnation, by using supercritical carbon dioxide to deliver anddeposit biocide into said wood. In the paper experiments are describedwhere wood samples of 100 mm or less are used for the impregnation.

U.S. Pat. No. 5,364,475 describes a process for removing chemicalpreservatives by extraction using super-critical carbon dioxide as theextractive medium and wood samples of a size of 10×50 mm.

WO 00/27601 discloses an impregnation of wood using supercritical carbondioxide where the pressure is released after the treatment in apulsating way in order to avoid or reduce resin exudation to the surfaceof the wood.

In the literature there have been reports of change of mechanicalproperties of wood samples treated under supercritical conditions.

In Anderson et. al. 2000, Forest Products Journal, 50:85-93, it isdescribed that the mechanical properties are affected by thesupercritical treatment. Western red ceder showed a reduction of up to23.1% of modulus of rupture and up to 13.7% reduction in modulus ofelasticity. Further it is described that some samples exhibited dramatictreatment defects and were spilt into hundreds of long slender sticks.It was further described that a relationship exists between sample sizeand damages to the wood, where larger samples are more damaged thatsmaller samples. The observed damages were allegedly caused by pressuregradients inside the wood.

SHORT DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a process for treatmentof wood susceptible to damage under high pressure, such as undersupercritical conditions, where damages are avoided or reduced. In aparticular preferred embodiment the wood being treated is having alength above a critical length.

In one preferred embodiment an object of the invention is a methodcomprising following steps:

-   -   a) a vessel is charged with wood to be treated;    -   b) the vessel is pressurized using the carrier fluid until the        treatment pressure is reached;    -   c) a holding period where the pressure is essentially constant        or the pressure changes at a low rate;    -   d) depressurising the vessel to ambient temperature followed by        removal of the treated wood.

The present inventors has realized that when the length of wood samplesincreases there is a certain length where the incidence of damage to thewood increases steeply. This length is defined in this description asthe critical length.

This object is achieved by a method for high pressure treatment wherethe fluid is not allowed to enter into the liquid phase at any place ortime during the process.

In one preferred embodiment the process is performed in a way so thetemperature of the wood to be treated does not exceed the plastificationtemperature of the wood during pressurizing of the vessel wherein thetreatment is to take place.

In another preferred embodiment the process is performed in a way so thetemperature of the wood to be treated does not exceed the plastificationtemperature of the wood during pressurizing and the de-pressurising ofthe treatment vessel.

The invention is based on the realization that fluids, suitable for highpressure treatments, in particular supercritical treatments, in liquidform has a significant lower penetration than in the gaseous orsupercritical state, and consequently may said fluid in liquid form betrapped inside the wood and cause the formation of excessive pressuregradients which may lead to damages of the wood.

Alternatively condensed liquid may undergo a significant volume changeif the temperature is increased subsequently during the pressure cycle.The latter may also cause significant pressure gradients and lead todamage.

Further it has been realized that when the temperature of the woodexceeds the plastification temperature the wood becomes susceptible todamage by even modest pressure gradients.

Thus one object of the invention is to provide a process for treatmentof wood using a carrier medium under high pressure in particular undersupercritical conditions, with avoidance of damage of the wood beingtreated.

In one embodiment the invention relates to a method for treatment ofwood susceptible to damage comprising the following steps:

-   -   a) a vessel is charged with wood to be treated    -   b) the vessel is pressurized with a fluid optionally containing        a dissolved active compound, until the treatment pressure is        reached    -   c) a holding period where the pressure is substantially constant    -   d) depressurising the vessel to ambient pressure        wherein the temperature is controlled in such a manner that the        fluid is not allowed to exist in liquid form in the wood.

Carbon dioxide is a preferred medium to be used as carrier mediumaccording the invention.

Another object of the invention is to provide a method for determining asuitable pressurizing and depressurising course for a treatment of wood.

Further objects of the invention are to provide methods forde-pressurizing a treatment chamber for treatment of wood bysupercritical carbon dioxide in order to avoid damage of the wood.

As the invention is concerned with the pressurizing and de-pressurizingof the vessel containing wood it applies to any process where wood istreated in a vessel using a fluid under high pressure, the inventionrelates in particular to impregnation, dying, drying and extractionprocesses.

In some embodiments raising the temperature above the plastificationtemperature may provide for a higher deposition and/or a better adhesionof the active compound in the wood. Such embodiments are alsocontemplated as being part of the invention.

It is important that the temperature raise above the plastificationtemperature is performed when no steep pressure gradients are present inthe wood, such as in the holding period of the treatment process.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a temperature-entropy diagram (TS-diagram) for carbondioxide where the border between the supercritical state and the liquidstate is indicated with a thick line.

FIG. 2 shows a TS diagram for carbon dioxide with indication of threedifferent paths A, B and C respectively, for depressurising.

FIG. 3 shows a TS diagram for carbon dioxide with a preferred path fordepressurising.

FIG. 4 shows a photograph of wood samples after treatment undersupercritical conditions where the pressure of the treatment vessel wasreleased without use of the precautions according to the invention. Thelength of the samples are from left to right 1.2 m, 1.0 m, 0.75 m, 0.5 mand 0.25 m.

DETAILED DESCRIPTION OF THE INVENTION

Treatment of wood using a carrier medium under high pressure has in therecent years been extensive explored in order to exploit the benefits ofcarrier media under high pressure. In particular carrier media undersupercritical conditions having penetration properties similar to a gasand solubility properties as a liquid have been explored.

The treatment procedure is usually divided in at least three differentfunctional steps or periods, a pressurizing step, where the pressure inthe treatment vessel increases from ambient pressure to the treatmentpressure; a holding period where the pressure is relative constant andwherein the compounds to be deposited in the wood are deposited; andfinally a depressurizing step where the pressure is decreased to ambientpressure again.

Even though the pressure during the holding period not necessary will beconstant the pressure variations and thus the pressure gradients in thevessel and inside the wood will be relative small compared with thesituations during pressurizing and depressurising.

Even though it is described that the deposition takes place during theholding period, it will be appreciated that some deposition may alsotake place during the pressurizing and depressurising steps.

In the present application the term carrier or carrier medium isintended to mean a fluid in form of a gas or in the supercritical stateused for the particular treatment. Depending on the particular use thecarrier will serve as solvent for the active compounds to be deliveredto the wood in case of a impregnation process or as solvent for thecompounds to be dissolved from the wood in case of an extractionprocess.

The carrier may comprise further components depending on the particulartreatment procedure to be performed, such as active components to bedeposited in the wood, cosolvents to facilitate the dissolution of theactive components or to facilitate extraction of particular componentsfrom the wood. The skilled person may suggest further components thatmay be comprised in the carrier.

In accordance with the present invention the treatment using the carriermedium takes place under high pressure. Carriers under high pressure orin the supercritical conditions are also in the literature occasionallycalled dense gases.

Even though the present description is mainly explained in relation tocarriers under supercritical conditions, the skilled person willappreciate that the conditions leading to damages in the treated woodsimilarly apply to carriers under high pressure but below supercriticalconditions, and therefore the present application relates to treatmentprocesses using a carrier media under high pressure regardless if thecarrier is in the supercritical state or not.

A process taking place under high pressure is according to the inventionintended to mean that the pressure in the treatment plant in at leastone period during the treatment process is significantly higher thatambient pressure. In particular the pressure is at least 20 bar aboveambient pressure, preferably at least 40 bar above ambient pressure,more preferred more that 60 bar above ambient pressure, and in aparticular preferred embodiment more that 80 bar above ambient pressure.

In a preferred embodiment the treatment takes place at a pressure in therange of 85-300 bar, preferably 100-200 bar, more preferred in the rangeof 120-170 bar and most preferred in the range of 140-160 bar.

The term “penetration” is according to the invention intended to meanthe property of the carrier fluid to enter into the wood being treated.Thus a fluid that enters into a compartment of the wood located at along distance from the surface has better penetration properties than afluid that under same conditions only enters into compartments of thewood located closer to the surface.

A fluid in the gaseous or supercritical state has significantly betterpenetration properties than same fluid in the liquid state.

The term “permeability” is according to the invention intended to mean aproperty of the wood describing the resistance against the penetrationof a fluid in said wood. Thus wood species having high permeabilityexerts a lower resistance to the penetration of a fluid than woodspecies having a low permeability.

The carrier medium to be used according to the invention may inprinciple be any suitable carrier having the desired dissolutionproperties for the intended use. It is preferred to use a medium havinga critical point at a low temperature and low pressure in order to avoidto high temperature or to high pressure. Thus a medium having a criticalpoint at a temperature of 20-50° C. and a pressure of 5-100 bars ispreferred.

Suitable carriers will be known in the art. It is within the skills ofthe average practitioner to select a suitable carrier for an intendeduse.

Examples of useable carriers according to the invention are known forthe person skilled in the art. Carbon dioxide is a preferred carrier.

The present invention prescribes measures for avoiding to damage woodduring a treatment under high pressure. Thus the skilled person willrealize that the invention may be used for any treatment of wood underhigh pressure.

The treatment may be an impregnation process where one or more activecompounds are deposited in the wood. These active compounds may bebiocides, fungicides, insecticides, colorants, fire retarding compounds,strength improving compounds etc.

The treatment may be an extraction process where particular compoundsare extracted from the wood, such as resin, terpenes etc., or it may betoxic compounds that have to be removed from wood before disposal of thewood.

In handling media at high pressures it is known that by raising thepressure of a gas the temperature will increase and by reducing thepressure the temperature will decrease. These properties are well knownfrom physical teachings describing behaviour of gases (such as theJoule-Thomson effect).

Consequently, the person skilled in the art will appreciate that acarrier medium used under high pressure conditions may during treatmententer into the liquid state because of a drop in temperature effected bya drop in pressure.

If the temperature drop inside the wood allows the carrier to exist inliquid form a dramatic change will happen. The liquid carrier having asignificant lower penetration in the wood will be trapped inside thewood and as the carrier is removed from the vessel a increasinglysteeper pressure gradient is formed from the inner of the wood where theliquid is trapped to the outside of the wood from where the carrier isremoved. This pressure gradient may eventually lead to a rupture anddamage of the wood if the wood is of a species that is susceptible todamage.

Contrary, if the carrier is maintained in a gaseous or supercriticalstate the penetration of the carrier in the wood is so high that it mayescape without damaging the wood or only damaging the wood in a muchlower extent.

Wood, as a natural material, is not very homogeneous and it will as itis known vary between different trees and sources due to differentweathering conditions, ground characteristics, genetic background etc.Further as the growth of trees is a multi annual process, the propertiesof a single wood sample may vary due to changing weather.

Therefore is wood inherently inhomogeneous and it will be possible tofind a sample of wood that will withstand a treatment that will damageanother sample, even though the two samples may originate from the samespecies.

The structure and architecture of wood is well known with long fibres inthe axial direction arranged in a characteristic pattern with annularrings. This structure leads to very different permeabilities in theaxial and the radial directions where the permeability in the axialdirection is significant larger that in the radial direction. It isassumed that the permeability in the axial direction is 10-20 fold ormore, greater than in the radial direction. However the invention is notintended to be limited to any particular theory.

Under pressurizing and depressurising it is believed that the carrierfluid enters into the wood and flows axially and/or radically throughthe wood, and that the cell walls of the wood form the observedresistance against the flow. Thus during pressurizing and depressurisinggradients are formed from cell to cell throughout the wood.

As a consequence of the dominating flow in the axial direction theperson skilled in the art will appreciate that steeper pressuregradients between the central part of wood and the outside will beformed in long specimens compared to shorter specimens.

This leads to the expectation that large wood pieces should be moresusceptible to damages during treatment using carriers under highpressure. In fact the connection between length of the specimens and thesusceptibility for damages can be observed experimentally.

One may define a critical length for specimens of a susceptible woodspecies, which define the length where susceptibility to damages occurs.In order to treat specimens above said critical length particularmeasures have to be taken in order to avoid damaging the wood whereasfor specimens below that length no such measures are needed. Due tovariation of wood within a batch such a limit may be broad. For useaccording to the invention the critical length is defined as the lengthwhere the susceptibility to damage in the batch is at an acceptablelevel e.g. damages are observed at a frequency of less than 5%,preferably less that 2%, where the frequency is understood as thefrequency of boards having one or more damages.

The critical length will vary between different species of wood and mayeven vary within one species depending of growth place and conditionsdetermined e.g. by the latitude where the particular wood has grown. Thecritical length for a given batch of wood may be determined byimpregnating samples having different lengths under supercriticalconditions, with fast and uninterrupted withdrawal of the supercriticalfluid after impregnation, followed by visual inspection of the samplesto establish the critical length above which the samples are damaged insevere degree.

For example can the critical length be determined by impregnating thesamples at a pressure of 85-150 bar and at a temperature of 40-60° C.,releasing the pressure down to 20 bar over 40-60′ minutes and finallyreleasing the pressure to 1 bar over 40 minutes, followed by visualinspection of the samples.

Typically critical lengths are found in the range, of 0.4-6 m, moretypically 0.5-3 m.

The critical length may vary with the water content of the wood. Furtherthe critical length may depend on the particular pressure temperatureprofile selected.

For example has it been established that for sprouse samples fromLilleheden, Denmark the critical length was found to be 1.2 m.

In absence of experimental data it can for practical purposes be assumedthat the critical length for a given batch is 1 m.

Different wood species show different susceptibility for damage duringtreatments with supercritical fluids. Some species are very resistant todamages whereas other species are susceptible. It is assumed that thefactors determining if a given wood species is susceptible or not liesin the structure of the wood even though the determining factor is notexplicitly known.

Wood species that are susceptible to damage is according to theinvention also called refractory species.

Thus the skilled person will appreciate from the teaching of the presentdescription that a wood article is a susceptible wood article if thearticle originate from a refractory species and the dimension of thearticle is so that the length of the article exceeds the critical lengthfor the particular species.

In order to determine if a wood species is a refractory specie, asuitable numbers of specimens thereof can be put under pressure usingthe carrier fluid in question and depressurised in a short time andsubsequently the specimens are examined for damages. For example samplescan be pressurized with carbon dioxide to 150 bars at 35° C. anddepressurised in 30 min and subsequently examined for damages. If thenumber of damages observed after this treatment is above the selectedlimit the wood is of a refractory species.

Examples of refractory species of wood according to the invention are:spruce, Western red cedar and Engelman Spruce.

According to the present invention damages of wood caused bysupercritical treatment can have different impact. The damages can beobserved as reduction of strength, reduction of elasticity, cracks,compressions or ruptures of the wood structure where the wood isfragmented into numerous long slender sticks. For the purpose of thisinvention there is no distinction between the various forms for damagesand they are all referred to simply as “damage”.

When a piece of wood pressurized by a carrier medium e.g. carbon dioxideis depressurised different temperature-entropy paths may be followeddepending on the location in the wood. In a central cell of the wood thedepressurising is assumed to be essentially iso-entrophic i.e. followinga path corresponding to path A in FIG. 2. In cells lying outside thecentral cells carbon dioxide is coming from the inner cells andsimultaneously carbon dioxide is flowing out of the cells in question,and therefore the depressurising is no longer iso-entropic but follows apath similar to path B or C in FIG. 2, where path B represent a celllocated closer to the central cell that the cell represented by path C.

As is can be seen from FIG. 2, path A and B crosses the thick lineseparating the liquid and the supercritical condition, and thereforewill liquid carbon dioxide be formed in this cell. The formation ofliquid carbon dioxide has dramatic consequences because the penetrationof liquid carbon dioxide in wood is significant lower that thepenetration of supercritical carbon dioxide in wood, and consequentlythe release of carbon dioxide from said cell is significant reduced. Asa consequence a very steep pressure gradient is formed between said celland the surroundings, which may lead to a rupture and damaging the wood.

According to the invention damaging during supercritical treatment ofwood susceptible to damage and having a length over the critical lengthis avoided or reduced by performing the treatment in a way so that thesupercritical fluid is not allowed to exist in its liquid form insidethe wood.

The person skilled in the art will know how to interpret aTemperature-Entropy diagram (TS-diagram) as shown in FIG. 1 or similar,and will know in which state the compound in question exists indifferent areas or the diagram. In particular he will know the borderbetween the supercritical state and the liquid state, which border mustnot be crossed according to the present invention. Thus the task for theperson skilled in the art is to select conditions and a path fordepressurising that do not cross said border.

The inventor has further realized that in addition to damages occurringbecause the fluid used as supercritical fluid is trapped in liquid forminside the wood, further damages may occur during pressurizing ordepressurizing of the wood. In particular it has been realized thatdamages occur during pressurizing or depressurizing dependent on thetemperature of the wood, where the wood will be more damaged if thetemperature is increased above the plastification temperature of thewood.

The plastification temperature of the wood is defined as the temperaturewhere the wood becomes deformable by small pressure differences. Theskilled persons will appreciate that the plastification temperatureaccording to the invention corresponds to the temperature needed fordeformation of wood using a usual steam box, and will further appreciatehow to determine such temperatures.

The plastification temperature may also be known as plastifization orsoftening temperature. In the present application these terms areconsidered equivalent.

For example for Nordic spruce having a moisture content in the range of16 to 23% it has been found that the plastification temperature isapproximately 50° C.-55° C.

Without wishing to be bound by any theory it is believed that when thetemperature of the wood exceeds the plastification temperature the woodbecome easy deformable and susceptible to deformation by even modestpressure gradients, which may lead to damages of the wood.

Thus in a preferred embodiment according to the invention wood istreated with a supercritical fluid where the temperature of the wood isnot exceeding the plastification temperature during pressurizing ordepressurizing.

In a particular preferred embodiment according to the invention theprocess is performed so that the fluid used as supercritical fluid isnot allowed to enter the liquid state inside the wood, and thetemperature of the wood is not allowed to exceed the plastificationtemperature during pressurizing and depressurising.

Raising the temperature to above the plastification temperature of thewood may according to the invention increase the deposition of thecompounds to be deposited. It is important that the pressure is notraised above the plastification temperature when large pressuregradients are present in the vessel in order to avoid damaging the wood.Therefore the raise of the temperature above the plastificationtemperature of the wood should not be performed before the holdingpressure is reached or until the rate of pressure increase has slowedconsiderable. The temperature should be lowered to below theplastification temperature before the depressurising is started.

Raising the temperature above the plastification temperature of the woodmay in some embodiments provide for a better deposition and a betteradhesion of active compounds in the wood, which leads to higherimpregnation efficiency and less leakage of the deposited activecompounds after the treatment.

In order to treat wood using a carrier under high pressure informationon the conditions inside the wood may be useful. This information,particular regarding temperature and pressure may be obtained by routineexperimentations, e.g. by insertion of probes into a suitable number ofpieces of wood and measuring the temperature and pressure during a testtreatment using a high pressure. The person skilled in the art willappreciate that a suitable number of samples should by used for thistest run, which number should be selected with regard to theheterogeneity of the batch etc.

The invention may also be used without detailed knowledge of theconditions inside the wood. The present application teaches thatconditions, where the carrier exists in a liquid form should not ariseinside the wood and the temperature of the wood should not exceed theplastification temperature of the wood during pressurizing anddepressurizing of the vessel. The skilled person will appreciate how toavoid such conditions. Measures that may be applied are measures thatlead to a move away from the border between supercritical fluid andliquid i.e. pressure and temperature, and preferably temperature, takingcare of not to exceed the plastification temperature.

In one preferred embodiment heat is supplied to the vessel, in which thesupercritical treatment takes place, during the de-pressurizing. Theamount of heat to be supplied must be sufficient to secure that theconditions inside the wood do not allow the fluid used in the treatmentto exist in a liquid form. For example if the supercritical fluid iscarbon dioxide a temperature above the critical temperature of 31° C.will secure that no liquid carbon dioxide occur.

The temperature may be measured by insertion of a temperature sensore.g. a thermometer in one or more specimens at each run or in arepresentative number of specimens during a test run.

The means available for the skilled person in order to adjust thetemperature during the treatment comprises addition of fluid, withdrawalof fluid, addition of heat, withdrawal of heat and any suitablecombination of these.

In one embodiment the heat is supplied to the vessel by feeding andwithdrawing supercritical fluid simultaneously where the temperature ofthe fluid being fed into the vessel is higher that the temperature ofthe fluid being withdrawn, and the amount of the fluid being fed is lessthat the amount being withdrawn.

In another preferred embodiment depressurising is performed by thefollowing steps:

-   -   (a) Removal of carrier from the vessel until a pressure and        temperature below the starting condition is reached;    -   (b) Addition of supercritical fluid having a temperature higher        than the fluid in the vessel until a pressure lower than the        starting pressure in step (a) and/or adding heat to the vessel;    -   c) Repetition of step (a) and (b) one or more times;    -   d) When the pressure is sufficient low releasing the pressure        until the approximately atmospheric pressure, and removal of the        treated wood.

The person skilled in the art will appreciate that during step (a), thetemperature will drop in the vessel and inside the wood samples.

In this embodiment a path in a TS diagram as outlined in FIG. 3 isfollowed where the path indicated is the condition in the centre of thewood.

The number and heights of each step (a), as well as the temperaturedifference between the fluids being withdrawn and being fed in step (b)can be determined by routinely experiments with regard to the TSdiagram. It is preferred that the number of steps is between 2 and 10most preferred between 3 and 6. The height of each step is preferablybetween 5 and 50 bars; more preferred between 10 and 30 bars and mostpreferred 15-25 bars.

One preferred method comprises following steps:

-   -   a) a vessel is charged with wood to be treated;    -   b) the vessel is pressurized using the carrier fluid until the        treatment pressure is reached;    -   c) a holding period where the pressure is essentially constant        or the pressure changes at a low rate;    -   d) depressurising the vessel to ambient temperature followed by        removal of the treated wood.

As preferred pressures and temperature can be mentioned:

-   -   A treatment, wherein the supercritical treatment pressure in        step c) is in the range of 85-300 bar, preferably in the range        of 100-200 bar, more preferred in the range of 120-170 bar and        most preferred in the range of 140-160 bar.    -   A treatment, wherein the temperature of the carrier fluid in the        wood is above 10° C., preferably above 20° C., preferably above        25° C., preferably above 30° C., more preferred above 32.5° C.        and most preferred above 35° C.    -   A treatment, wherein the temperature of the carrier in the wood        is in the range of 25-65° C., preferably in the range of        31-55° C. in step b) and d) when the pressure is above 30 bar.    -   A treatment, wherein the temperature during step b) and d) is        below 65° C., preferably below 60° C., preferably below 55° C.,        more preferred below 50° C. and most preferred below 45° C.    -   A treatment, wherein the temperature during step d) is above 45°        C., preferably above 50° C., preferably above 55° C. and more        preferred above 60° C. when the pressure is above 30 bar.

When the pressure has been reduced to 10-30 bars it may be released toatmospheric pressure without further measures (step (d)).

It will be understood that damages may also be avoided by reducing thespeed of pressure reduction. In this way the depressurising will takelonger time which secures a better heat distribution and consequentlythe temperature in the centre is not allowed to drop as much as if thepressure was released with a higher speed. Further more time will beavailable for the supercritical fluid inside the wood to flow out of thewood. Consequently the formed pressure gradients will be less steep.However from an industrial point of view this solution is not attractivebecause the longer time used for pressure reduction means that eachbatch occupy the plant for a longer period of time, which again meansthat the productivity of the plant is reduced.

The invention is now further illustrated by the following examples,which are intended as illustration of the invention and should not beregarded as limiting in any way.

EXAMPLES Example 1

Determination of the Critical Length

For this determination Sprouse wood samples obtained from Lilleheden,Denmark, were used.

Samples having lengths in the range of 0.25 to 1.2 m were impregnatedunder supercritical conditions using carbon dioxide as the solvent.

The samples were impregnated at a temperature of 55° and a pressure of150 bar using 50 g biocide corresponding to a deposition of 0.25 kgbiocide per m³ wood.

After impregnation for 20 minutes the pressure in the vessel wasreleased according to the courses listed in table 1. below.

When the pressure reached atmospheric pressure the vessel was opened andthe samples removed and inspected visually for damages. TABLE 1 Length(m) 0.25 0.25 0.5 0.75 1.0 1.2 1.2 Time for Pressure 20 40 20 20 20 2040 release 150-85 bar (min) Time for pressure 40 60 40 40 40 40 60release 85-20 bar (min) Time for pressure 40 40 40 40 40 40 40 release20-1 bar (min) damaged no no no no no yes yes

From the results it appears that the critical length of the wood is 1.2m. Further it can be seen that slowing the pressure release and therebyextending the time for pressure release 150 −85 bars from 20 to 40minutes and 85-20 bar from 40 to 60 minutes did not change the criticallength.

In FIG. 4 the samples are shown where it is obvious that the sample of1.2 m is severely damaged, whereas the other samples are not damaged.

Example 2

Effect of Temperature on the Number of Damages

This example demonstrates the dependence of the temperature on thenumber of damages.

In a vessel were common 1″×3″ spruce boards treated using supercriticalcarbon dioxide as the medium. The boards had a total length of 1,5 m. Inorder to mimic boards having a length of 3 m boards were blinded in oneend so that the medium was only capable of entering the boards in oneend.

Different pressures, temperatures and depressurising times were selectedas indicated in table 1, and the damage rate was calculated as thepercentage of boards having one or more damages. TABLE 2 LengthTemperature Time for depressurising Rate of damage (m) (° C.) (min.) (%)1.5 45 39 22 3 45 39 41 3 55 39 13 3 65 39 5 3 55 90 7 3 65 90 0

From the data in table 2 it can be deduces that the length of the boardshas a significant influence on the damage rate from 22-41 percent within increase in length from 1.5 to 3 m.

Further it can be deduced that the treatment temperature i.e. theinitial temperature before depressurising, has a marked influence on thedamage rate. At a depressurising time of 39 minutes a decrease from 41%to 5% at a temperature from 45° C. to 65° C. respectively, can beobserved.

A similar decease can be observed for a time for depressurising at 90minutes, now on a lower level because of the longer time.

1. A method of treatment of wood susceptible to damage using a carrierfluid under high pressure characterized in that the fluid is not allowedto exist in its liquid form inside the wood.
 2. The method according toclaim 1, wherein the treatment is selected among impregnation,extraction, dying or drying.
 3. The method according to claim 1, whereinthe method comprises: a) charging a vessel with wood to be treated; b)pressurizing the vessel using the carrier fluid until the treatmentpressure is reached; c) providing a holding period in which the pressureis essentially constant or the pressure changes at a low rate;depressurizing the vessel to ambient temperatures, followed by removingthe treated wood.
 4. The method according to claim 3, wherein thetreatment is an impregnation and the active component is added to thesystem in a) or b).
 5. The method according to claim 4, wherein thetreatment takes place at least partially in the supercritical state. 6.The method according to claim 5, wherein the supercritical treatmentpressure in c) is in the range of 85-300 bar, preferably in the range of100-200 bar, more preferred in the range of 120-170 bar and mostpreferred in the range of 140-160 bar.
 7. The method according to claim5, wherein the temperature of the carrier fluid in the wood is above 10C, preferably above 20 C, preferably above 25 C, preferably above 30 C,more preferred above 32.5 C and most preferred above 35 C.
 8. The methodaccording to claim 7, wherein the temperature of the carrier in the woodis in the range of 25-65 C, preferably in the range of 31-55 C in b) andd) when the pressure is above 30 bar.
 9. The method according to claim3, wherein the temperature of the wood is below the plastificationtemperature of the wood during b).
 10. The method according to claim 3,wherein the temperature during b) and d) is below 65 C, preferably below60 C, preferably below 55 C, more preferred below 50 C and mostpreferred below 45 C.
 11. The method according to claim 3, wherein thecarrier comprises carbon dioxide.
 12. The method according to claim 3,wherein heat is extracted during d).
 13. The method according to claim3, wherein heat is added during b).
 14. The method according to claim 1,wherein the wood susceptible to damage is selected among a refractorywood species.
 15. The method according to claim 14, wherein the woodsusceptible to damage is selected among: spruce, Engelman spruce andwestern red cedar.
 16. The method according to claim 3, wherein thetemperature is raised above the plastification temperature during c).17. The method according to claim 1, wherein the temperature during d)is above the plastification temperature when the pressure is above 30bar.
 18. The method according to claim 17, wherein the temperatureduring d) is above 45 C, preferably above 50 C, preferably above 55 Cand more preferred above 60 C when the pressure is above 30 bar.
 19. Themethod according to claim 3, wherein the depressurizing is performed by:i) removing of fluid from the vessel until a pressure and temperature inthe supercritical state but below the starting condition is reached; ii)adding and/or heat until a pressure that is lower than the startingpressure in i) is reached; iii) repeating i) and ii) one or more times;iv) when the pressure is sufficiently low, releasing the pressure untilapproximately atmospheric pressure is reached, and removing the treatedwood.
 20. The method according to claim 1, wherein the wood is spruceand the temperature is higher than 31° during the process and lower than55° during the pressurizing and depressurising.
 21. An article of woodtreated according to claim
 1. 22. A method for establishing theconditions for a treatment of wood susceptible to damage having a lengththat is over the critical length with a supercritical fluidcharacterised in that the treatment is conducted in a way so thecritical fluid is not allowed to exist in its liquid form inside thewood.
 23. The method according to claim 22, wherein the temperature ofthe wood during pressurizing and depressurizing is below theplastification temperature of the wood.